The present invention relates to a lens meter for measuring optical properties of a spectacle lens or a contact lens.
When optical properties of a lens are measured by a manual lens meter, the image focusing plane is obtained by crude visual observation or by enhanced observation using an apparatus. Relative positions of elements in the optical system, including the position of the examined lens along the optical axis and the distance between the examined lens and the optical system having a light source, are varied; and the focal length is obtained by determining the distance required to move the elements to find the image focusing plane and the distance measured between the examined lens and the optical system having the light source. However, obtaining the image focusing plane by crude or enhanced visual observation has a drawback in that an error in measurement tends to occur depending on the individual persons performing the measurement. This error may include random human errors and systematic measuring errors if the observer""s vision includes a refractive error.
To overcome the above drawbacks inherent in the use of the manual lens meter, automatic lens meters have been proposed as described in Japanese Patent Application Laid-Open Nos. Showa 49(1974)-122355, Showa 60(1985)-17335 and Heisei 8(1996)-20334. An automatic lens meter described in Patent Application Laid-Open No. Showa 49(1974)-122355 comprises an examined lens positioned so that the optical center of the examined lens is placed on the optical axis of a collimator for sending light and a collimator for receiving light, an optical system for taking an image which has a photoelectric converter disposed on a focusing plane along the extension of the above optical axis, an electric circuit which arranges luminance signals of a target image decomposed by scanning lines of the photoelectric converter and calculates the position where the time width area of the electric luminance signal between one direction of movement and another direction of movement of the optical system having a light source is minimized, and a circuit automatically controlling the reciprocal movement of the optical system having a light source synchronously with the above electric circuit by a pulse motor and a signal generator for driving the pulse motor. The position where the image of the target has the smallest area, and is most focused, is converted into a signal expressed as the diopter.
In automatic lens meters, including the automatic lens meters described in Japanese Patent Application Laid-Open Nos. Showa 49(1974)-122355 and Showa 60(1985)-17335, it is necessary that a means for detecting the best position and a mechanism for moving the target be disposed. Therefore, an improvement is made when the refractive index, the angle of the axis and the prism values of the lens for examination can be measured by treatments of signals of an image sensor disposed on the focusing plane of an image-forming lens without the need to move a slit pattern. Such a simplified construction is disclosed as the automatic lens meter described in Japanese Patent Application Laid-Open No. Heisei 8(1 996)-20334.
In the automatic lens meter described in Japanese Patent Application Laid-Open No. Showa 60(1985)-17335, the incident light rays used for the measurement of optical properties are divided into two groups of component rays by the object lens that received the incident rays. The decentering distance of the rays measured or focused by the examined lens, being related to the prism value of the examined lens, is detected by two image sensors, and the refractive power is calculated from the data obtained by the two sensors. Since the measured rays, having passed through the lens being examined, are divided into two groups of refracted rays, being a group of rays refracted or polarized in the X-direction and a group of rays refracted or polarized in the Y-direction, sent to the image sensors, the amount of the light signal sent to each image sensor decreases to one half or less of the original intensity of the incident light signal. Therefore, a special electric treatment for increasing the S/N ratio is necessary but the electric circuits required to perform the amplification are complicated and expensive.
Moreover, because a beam splitter for dividing the rays and the two image sensors are required, it is inevitable that to construct the path of rays, the two image sensors and portions for disposing these components make the apparatus complicated and expensive to manufacture.
In the automatic lens meter described in Japanese Patent Application Laid-Open No. Heisei 8(1996)-20334, the decentering distances of the rays measured or focused by the examined lens in the X-direction and in the Y-direction are detected by a single image sensor placed at a position behind the object lens that receives the measured rays, and the refractive power of the examined lens is calculated from the data obtained by this single sensor. A slit pattern having an xe2x80x9cNxe2x80x9d shape is used when performing the measurement; therefore, a portion of the slit pattern is placed obliquely with respect to the image sensor to detect rays in the Y-direction and this causes a problem in that the accuracy of the information on the position decreases.
The present invention overcomes the above problems and has an object of providing an automatic lens meter having a simple structure and exhibiting a high accuracy of measurement.
The present invention provides an automatic lens meter comprising an optical system in which a pattern image is formed by projection of a pattern created in a pattern creating member. Then, the optical properties of the lens being examined are measured based on the displacement of the pattern image from a base position wherein the path of the rays in the optical system does not include the examined lens to a measuring position wherein the path of the rays in the optical system includes the presence of the examined lens. The automatic lens meter comprises a plurality of light sources arranged at positions separated from an optical axis of the optical system by a same distance (also referred to as xe2x80x9cequidistantxe2x80x9d) on a plane perpendicular to the optical axis, a light transfer lens which is disposed in a manner such that an optical axis thereof is the same as the optical axis of the optical system and arranges rays from the light sources into parallel rays, a measurement table on which the lens for examination is disposed in a path of rays from the light transfer lens, a collimator lens through which rays from the light transfer lens pass and form images of the plurality of light sources on the lens for examination disposed on the measurement table, a pattern creating member which is disposed between the light transfer lens and the collimator lens, an object lens for receiving rays which forms a pattern image of the pattern created by the pattern creating member on a specific pattern image forming plane, and line sensors arranged on the pattern image forming plane, wherein the pattern image is formed as a tetragonal shape on the pattern image forming plane from the pattern created in the pattern creating member, the line sensors are arranged so as to intersect four sides of the tetragonal shape of the pattern image on a same plane as the pattern image forming plane, and the information on displacement of the pattern image is obtained from information on positions of intersections of the line sensors and the four sides of the tetragonal shape of the pattern image.
Further objects, features and advantages the present invention will become apparent from the Detailed Description of Preferred Embodiments which follows when considered together with the attached Drawings.